1. Field of the Invention
The present invention relates to a frequency error detecting circuit and method and a frequency correcting circuit configured to enable quick carrier frequency acquisition with a small circuit size.
2. Description of the Related Art
In a general broadcast system, a received radio signal is converted into a low-frequency signal or a baseband signal by a tuner functioning as an analog circuit in a high-frequency (RF) band, A/D-converted, and then subjected to demodulation and decoding processing. In the tuner, since VCOs (Voltage Controlled Oscillators) used in a transmitter and a receiver are different, a carrier frequency offset (hereinafter referred to as “frequency error”) occurs between the transmitter and the receiver. The carrier frequency error causes catastrophic errors in digital demodulation processing. Therefore, in the receiver, AFC (Automatic Frequency Control) for correcting the carrier frequency error is adopted.
In general, in a broadcasting transmitter and a communication base station, a high precision oscillator with a small frequency error is used. On the other hand, on the receiver side, a small and inexpensive oscillator is often used. Frequency accuracy of such an oscillator is relatively low. Because of the low frequency accuracy, depending on adjustment of the tuner, the carrier frequency error is relatively large with respect to signal bandwidth between the transmitter and the receiver. Therefore, it is desirable that the AFC can correct the error even when the relatively large carrier frequency error occurs in this way.
As a technique for correcting such a carrier frequency error, there is a technique disclosed in Japanese Patent Application Laid-Open Publication No. 2006-108817 (Document 1) and Japanese Patent Application Laid-Open Publication No. 2005-160116 (Document 2). The technique disclosed in Documents 1 and 2 is a technique for extracting a known signal sequence included in a received signal and detecting an inter-symbol phase difference using characteristics (repetition, uniqueness, etc.) of the extracted signal sequence to detect a frequency error.
However, the technique disclosed in Documents 1 and 2 cannot be used in a received signal not including a known signal sequence. When the technique is adopted in an apparatus configured to receive a broadcast wave unknown whether it is a single carrier system or a multi-carrier system, it takes long time to estimate the frequency error.
Meanwhile, Heinrich Meyr, Marc Moeneclaey and Stefan A. Fechtel, “Digital Communication Receivers,” A Wiley-Interscience Publication, pp. 453-456 (Document 3) discloses a technique for detecting a carrier frequency error according to a spectrum analysis method without using a known signal sequence. In the technique, a received signal is subjected to Fourier transform to observe a frequency spectrum and a shift of the frequency spectrum from a target center frequency is measured to detect a frequency error. By using such a spectrum analysis method, it is possible to detect a carrier frequency error even when a known signal sequence is not included in a received signal.
However, in the technique disclosed in Document 3, the detection range of the frequency error and the frequency resolution depend on the number of points of a Fourier transform circuit. When it is attempted to set a sufficient frequency detection range while enabling error detection at sufficient frequency resolution, it is necessary to increase the number of points of a Fourier transform circuit, for example, to 4096. Also the large number of points of the Fourier transform increases a circuit size and requires a large number of samples for error detection. Therefore, it takes long time to capture the number of samples required for the error detection and acquisition time for frequency synchronization is extended.